Cisco CCNA Study Guide - Router Alley

CCNA Study Guide v2.62 – Aaron Balchunas
10
OSI Model - The Upper Layers
The top three layers of the OSI model are often referred to as the upper
layers:
• Layer-7 - Application layer
• Layer-6 - Presentation layer
• Layer-5 - Session layer
Protocols that operate at these layers manage application-level functions,
and are generally implemented in software.
The function of the upper layers of the OSI model can be difficult to
visualize. Upper layer protocols do not always fit perfectly within a layer,
and often function across multiple layers.
OSI Model - The Application Layer
The Application layer (Layer-7) provides the interface between the user
application and the network. A web browser and an email client are
examples of user applications.
The user application itself does not reside at the Application layer - the
protocol does. The user interacts with the application, which in turn interacts
with the application protocol.
Examples of Application layer protocols include:
• FTP, via an FTP client
• HTTP, via a web browser
• POP3 and SMTP, via an email client
• Telnet
The Application layer provides a variety of functions:
• Identifies communication partners
• Determines resource availability
• Synchronizes communication
The Application layer interacts with the Presentation layer below it. As it is
the top-most layer, it does not interact with any layers above it.
(Reference: http://docwiki.cisco.com/wiki/Internetworking_Basics)
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